The present invention relates to an optical encoder for detecting the position, the movement speed, the moving direction and the like of a moving object with use of a light receiving element, and more specifically relates to an optical encoder which is preferably used for, for example, copying machines, printers or other printing equipment, as well as FA (Factory Automation) equipment.
JP H6-7013 U discloses a conventional optical encoder which has an encoder plate with an index track placed in the center and an incremental track placed on the outer circumference. This optical encoder stabilizes an index channel signal obtained from the index track and detects a reference position of the encoder plate with the stabilized index channel signal, while detecting movement of the encoder plate with an incremental channel signal obtained from the incremental track.
JP S58-37515 A discloses that two slits are placed on a code disc at an uneven interval so as to obtain an index channel signal with a high peak value. In the technique disclosed in JP S58-37515 A, a series of scales having a light-ON section and a light-OFF section include an index pattern in addition to an incremental pattern. The technique in JP S58-37515 A makes it possible to reduce a light emitting area and a light receiving area of the optical encoder.
In an optical encoder disclosed in JP 2007-64981 A, a moving object has a series of scales which include an incremental pattern and an index pattern, and an index channel signal is generated by logically combining outputs from at least three photodiodes in a photodiode array which outputs incremental channel signals as a movement signal of the moving object.
In the above-mentioned optical encoder in JP H6-7013 U, the index track is placed in the center of a light receiving section in order to detect an index channel signal with sufficient precision for origin detection (reference position detection). Thus, in the method for obtaining an index channel signal by placing the index track away from the incremental track, not only an optical area to use is widened, but also it is necessary to produce parallel beams corresponding to the wide optical area as well as to secure a light receiving area, which results in cost increase.
In the above-mentioned optical encoder in JP S58-37515 A, the index channel signal different in peak value from the incremental channel signal causes the incremental channel signal to have phase distortion and shift, and thereby movement detecting characteristics may be deteriorated.
In the above-mentioned optical encoder in JP 2007-64981 A, when the photodiode array which outputs an incremental channel signal is used as it is for obtaining an index channel signal, the phase shift of the incremental channel signal increases upon incidence of light into the index pattern due to pattern difference between an incremental pattern and an index pattern, which may deteriorate movement detecting characteristics.
An optical encoder disclosed in JP 2003-294494 A is composed of a plurality of light receiving elements with different light receiving surfaces including photodiodes ZPD 1 to 4 for origin detection and a circuit for processing detection signals from these light receiving elements, the circuit including a capacitor for correcting the detection signals, and the optical encoder detects a differential signal with a differential amplifier to detect the position of an origin. However, in JP 2003-294494 A, the light receiving surface areas of the photodiodes ZPD 1 to 4 for origin detection are different from the light receiving surface areas of photodiodes ZBPD 1 to 4 for non-origin detection, and therefore even with correction performed, variations in light receiving amount and the correction is not always correlated, which may prevent accurate detection of the position of the origin.
In an optical encoder in JP 2005-61896 A, n signals representing the amount of change of a moving object is obtained by providing the moving object with m slits arranged in the moving direction, arranging k photodiodes in a row wherein k is a common multiplier of n and m, and adding together output signals from a plurality of photodiodes among k photodiodes to obtain n movement information signals representing the amount of change of the moving object.
Further, an optical encoder disclosed in JP 2006-84458 A has photodiodes arranged, the number of the photodiodes being expressed as a product of the number of independent movement information signals with a power of the number of light transmission areas facing a light receiving section.
Although the light receiving surface of each photodiode is reduced and the photodiode is subdivided to obtain precise movement information signals in JP 2005-61896 A and JP 2006-84458 A, the index channel signal for detecting the reference position of a moving object is not obtained by these technologies.